Revert "Remove Expect from core library."

tests/corelib/date_time_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import "package:expect/expect.dart";
-
 // Dart test program for DateTime.
 
 class DateTest {
   // Tests if the time moves eventually forward.
   static void testNow() {
     var t1 = new DateTime.now();
     bool timeMovedForward = false;
     for (int i = 0; i < 1000000; i++) {
       var t2 = new DateTime.now();
       if (t1.millisecondsSinceEpoch < t2.millisecondsSinceEpoch) {
@@ skipping 806 matching lines @@
     testEquivalentYears();
     testExtremes();
     testFarAwayDates();
     testWeekday();
   }
 }
 
 main() {
   DateTest.testMain();
 }